The field of the invention is the manufacture of hydrogen chloride and the present invention is particularly concerned with gaseous hydrogen chloride purification from dilute aqueous hydrochloric acids.
The state of the art of the manufacture and processing of hydrochloric acid may be ascertained by reference to the Kirk-Othmer, "Encyclopedia of Chemical Technology" 2nd ed., vol. 11 (1967) pp. 307-337, particularly pp. 314-315 where it is disclosed that 75% of U.S. hydrochloric acid production for the years 1961-1966 came from chlorination processes, and pp. 315-323 under the section Gas Treatment and West German published application No. 2,633,640 corresponding to U.S. Pat. No. 4,115,530, the disclosures of which are incorporated herein.
In many industrial processes, hydrogen chloride arises as a by-product in the form of dilute aqueous hydrochloric acids, for which there is only a limited industrial use. In order to be able to utilize the hydrogen chloride, present in the form of a dilute, aqueous hydrochloric acid, as a chemical raw material, for example for the preparation of vinyl chloride, ethyl chloride, chloroprene or chlorosulphonic acid, the hydrogen chloride must be isolated as an anhydrous gas.
The working up of dilute acids by distillation, required to achieve this, is ruled out for reasons of economy because the amounts of energy which have to be provided to evaporate the water are too great. Extractive isolation of the HCl gas has hitherto failed because of the unfavorable partition coefficients between HCl and extractants, such as, for example, pentanols. Relatively long-chain amines, which must be water-insoluble both as such and in the form of their hydrochlorides, suggest themselves as extractants having far more advantageous partition coefficients. Using such amines, it is possible to separate off the hydrogen chloride almost completely from a dilute hydrochloric acid.
The subsequent thermal cleavage of the amine hydrochlorides thus obtained, and the isolation of the anhydrous hydrogen chloride, can then be carried out in accordance with the processes described in West German published application No. 2,633,640 and in U.S. patent application Ser. No. 10,048.
U.S. Pat. No. 4,115,530 discloses:
(a) that aqueous hydrochloric acid is extracted with an amine or a mixture of an amine and an inert, water-immiscible solvent which boils below the amine used, the latter being tertiary alkylamines, tertiary aryldialkylamines, secondary arylalkylamines, primary alkylarylamines or mixtures thereof, which contain 14 to 36 carbon atoms in the nitrogen bonded side chains, which side chains include at most one nitrogen-bonded methyl group and at least one aliphatic radical containing at least 6 carbon atoms, the acid constant K.sub.a of the amine being less than 10.sup.-3,
(b) that an inert, water immiscible solvent which boils below the amine used is added to the extract, unless the solvent has already been added in stage (a),
(c) that the extract is distilled, the resulting vapors are condensed, the water is continuously removed from the two-phase condensate, and the organic phase is returned to the distillation process, and
(d) that after removing the water, the extract is distilled under reflux at temperatures of between 100.degree. and 250.degree. C. at the column bottom, and the gaseous hydrogen chloride liberated at the top of the column is taken off.
According to the process of U.S. patent application Ser. No. 10,048, which does not form part of the prior art, the gaseous hydrogen chloride is obtained--in a modification of the process of U.S. Pat. No. 4,115,530--by heating the mixture of amine hydrochloride and solvent to a temperature below the boiling point of the solvent and separating off the hydrogen chloride liberated by passing an inert gas stream through the mixture.
U.S. patent application Ser. No. 10,048 discloses a method of manufacturing hydrogen chloride from solutions of amine hydrochlorides comprising:
(a) heating the solutions in an inert, organic essentially non-polar solvent at temperatures of about 120.degree.-230.degree. C., the solvent having a boiling point at least 20.degree. C. above the temperature of heating;
(b) simultaneously with the heating, passing an inert gas stream through the heated solutions of (a) to split off hydrogen chloride and produce a mixture of hydrogen chloride and the inert gas; and
(c) separating the hydrogen chloride from the mixture of (b) wherein the amine components of the amine hydrochlorides are tertiary alkylamines, tertiary aryldialkylamines, secondary arylalkylamines, primary alkylarylamines or mixtures thereof, each of the amine components containing 14 to 36 carbon atoms in the side chains bonded to nitrogen, not more than one of the side chains being a methyl group bonded to nitrogen and at least one of the side chains being an aliphatic radical, bonded to nitrogen, containing at least 6 carbon atoms.
It is true that these two processes mentioned for the first time permit the isolation of gaseous hydrogen chloride from dilute aqueous hydrochloric acid by means of special amines and subsequent thermolysis of the amine hydrochlorides formed, but they still suffer from shortcomings. Thus, the principal shortcoming is that the tertiary aliphatic amine decomposes, dependent on its structure and dependent on how high the temperature to which it is exposed. The decomposition takes place within a more or less short period, inter alia with the formation of primary and secondary aliphatic amines and alkyl chlorides, and the efficiency of the cleavage increasingly declines as a result thereof, since, though primary and secondary aliphatic amines extract hydrogen chloride effectively from dilute hydrochloric acid, their hydrochlorides are barely capable of thermal cleavage. There is therefore an initially insignificant formation of the interfering by-products, which however progressively becomes greater as the decomposition proceeds.
The process for isolating inexpensive hydrogen chloride by extracting dilute hydrochloric acid with amines however only operates economically if the relatively expensive amine can be recycled as far as possible without losses.